There has been conventionally known a technique for deforming a core material so as to form a stator core for a motor. In other words, in fabricating this kind of stator core, the yield of a magnetic steel sheet during fabrication is increased by the following technique. First, when a core material is punched from a magnetic steel sheet, not an annular core material but a belt-like core material is punched. At this time, the core material is constituted of a plurality of core pieces continuous to each other in a belt-like manner. The punched core material is deformed into an annular shape, so that a stator core is obtained.
Specifically, a predetermined number of core pieces, each having a tooth portion and a yoke portion, are punched from the magnetic steel sheet in the state in which they are connected to each other via a connecting portion. The connecting portion is plastically deformed to be bent, thereby obtaining an annular stator core. At this time, the connecting portion has a notch portion and a through hole elongated in a connection direction, and therefore, the connecting portion is likely to be plastically deformed (see, for example, PTL 1). Alternatively, the through hole may be formed into a substantially circular shape (see, for example, PTL 2).
FIG. 10 shows the configuration of conventional core material 91 disclosed in PTL 1. As shown in FIG. 10, core material 91 is constituted of a plurality of core pieces 92 connected to each other at thin portion 97 serving as a connecting portion. Core piece 92 includes tooth portion 95 and yoke portion 96. Moreover, core material 91 includes notch portion 93 having joint surfaces 93a and 93b between core pieces 92. At the tip of notch portion 93 is formed expanded through hole 94.
However, with the above-described conventional configuration shown in FIG. 10, through hole 94 is elongated in the connection direction, and further, thin portion 97 is long in the connection direction in parallel to through hole 94. Therefore, when the core material is deformed into an annular shape, thin portion 97 does not have any definite bending center. Thus, when thin portion 97 is plastically deformed to be bent, the bending center cannot be located at the same position at each of the connecting portions according to force exerted on each of yoke portions 96, thereby inducing the fear of mismatch between joint surfaces 93a and 93b of the notch portion 93. In this manner, with the conventional configuration, an inconstant joint state at each of the connecting portions raises the problem of degradation of circularity of a tooth.
In the meantime, with the configuration in which a through hole is formed into a substantially circular shape, as disclosed in PTL 2, the bending center of each of connecting portions is constant, but the region of a thin portion becomes narrow. In view of this, a machining force for forming a core material into an annular shape becomes large, thereby raising problems of an increase in iron loss due to an increase in compressive stress to be exerted on a joint surface and the degradation of motor efficiency caused by the iron loss.
PTL 1: Unexamined Japanese Patent Publication No. H9-308143
PTL 2: Unexamined Japanese Patent Publication No. H11-289695